older speaker connectors

What model speakers are we talking about? The cost of the upgrade with Cardas connectors may cost more than your speakers are worth. There are a lot of connectors available at reasonable prices.
Check this site out for some reasonable terminals. I've used them a couple of times, good service. Changing the posts is really not a diffivult little project.

If the labour doesn't cost alot, why not? It most likely will make them sound better. But then...you'll need to change the wire conectors and then you'll want to upgrade your amp, source etc.. Before you know it you'll be spending a fortune and you'll never get the pleasure out of your system as you did before you started all this. (BTW, BP Hex's are excellent binding posts just in case you decide to change them out.)

The labor is the real killer. If you cannot do it yourself, forget about it. Any Infinity speakers with spring clips are relatively cheap speakers! (hey, I used to own Infinity RSIIa $3K in 1983)
The connectors are not gonna make enough difference for the cost of the work. Better speaker wire, or better cables can do more. (and changing the connectors is not going to increase the used value of your speakers)